1. Field of the Invention
The present invention relates to a temperature-humidity exchanger that heats and humidifies a low-temperature dry gas by heat and moisture which are transferred from a high-temperature moist gas via a permeable membrane permeable to moisture, and more particularly to a fuel-cell temperature-humidity exchanger that heats and humidifies unreacted gases by heat and moisture transferred from an exhaust gas in a fuel cell.
2. Description of the Related Art
A conventional temperature-humidity exchanger is provided with a laminated body that is gas-tight against the outside. This laminated body is obtained by alternately and gas-tightly stacking frame members and permeable membranes in the vertical direction of the frame member. The frame members have an internal space that is open at the top and at the bottom and surrounded by a peripheral frame having a gas inlet and a gas outlet. A fitting protrusion is provided on the side of one of the vertically stacked frame members, and a fitting recess is provided on the side of the other. Gas sealing performance is ensured by fitting the fitting protrusion into the fitting recess. The laminated body has four lateral faces. One of them serves as the moist gas inlet side and the one opposite thereto serves as the moist gas outlet side. Another one of them serves as the gas inlet side and the one opposite thereto serves as the dry gas outlet side. Disposed on each of the four lateral faces of the laminated body is a corresponding one of gas supply and exhaust manifolds. The gas supply and exhaust manifolds are constructed as a rectangular parallelepiped having an open face, against which a sealing face as a projecting portion of the laminated body is pressed via a gasket. Thus, the interiors of the manifolds are connected to the internal space of the adjoining laminated body while gas-tightness is maintained.
However, the sealing face on the side of the laminated body, which is obtained by joining the fitting protrusion and the fitting recess together, is inferior in smoothness and causes a problem in that good sealing performance cannot be guaranteed with ease. The gas supply and exhaust manifolds and a mechanism for clamping them are required, which raises a problem of an overall increase in cost (e.g., see JP 2003-314983 A).
Thus, there has been proposed a temperature-humidity exchanger adopting an internal manifold design which allows gases to flow in a countercurrent manner and has a gas manifold provided in a gas separator. The gas separator is composed of a frame member forming an outer peripheral seal portion and a mesh plate forming a gas channel. The temperature-humidity exchanger, which is constructed of a laminated body obtained by disposing permeable membranes on top and bottom faces of the gas separator and further disposing gas separators on top and bottom faces of the permeable membranes, carries out exchange of heat and moisture between a moist gas and a dry gas via the permeable membranes. In the internal manifold design, since the gas manifold is provided in the gas separator itself, there is no need to provide gas seal between the manifold and the laminated body.
However, although the gas manifold and the permeable membrane are gas-sealed by a gap between the permeable membrane and surfaces of the mesh plate and the frame member surrounding the gas manifold, a dimensional difference in level or a material difference in elasticity or thermal expansion coefficient causes a problem of insufficient gas seal. The mesh plate, which is made of a metal or a polymer, has an uneven surface and thus causes a problem of a further deterioration in gas sealing performance. If an attempt is made to solve this problem by providing a sealing sheet capable of maintaining smoothness, the additional necessity of this sealing sheet entails structural complication. This creates another problem of expensiveness.
Thus, the gas manifold is surrounded by a seal portion integrated with the frame member so as to be gas-sealed by a gap between the seal portion and the permeable membrane. The gas separator is also provided with an underdrain as an entrance which establishes communication between the gas manifold and the mesh plate (e.g., see JP 2000-164229 A).
However, the gas separator, which is provided with the underdrain perpendicular to the laminating direction of the gas separator, cannot be easily mass-produced at a low cost by resin molding. Namely, resin molding includes the steps of preparing a split mold that can be split along one direction of a molded product, injecting liquid resin into the mold, splitting the mold after the resin has been solidified, and taking the molded product out. Therefore, the mold for gas separators cannot be easily provided with a portion corresponding to the underdrain perpendicular to the laminating direction of the gas separator. Thus, the necessity to machine the underdrain through post-machining leads to a problem of high cost.
Also, the thickness of the seal portion surrounding the underdrain in the laminating direction of the gas separator needs to be equal to or greater than a predetermined value with a view to guaranteeing the function of gas seal. For instance, the height of the underdrain with a rectangular cross section in the laminating direction is set as 1.5 mm to hold the pressure loss equal to or below a permissible pressure, and the predetermined thickness of the seal portion in the thickness direction is set as 1.25 mm to prevent a serious deformation from being caused by a pressure acting between the seal portion and the permeable membrane. Therefore, the gas separator is 4 mm thick. Since the mesh plate of the gas separator is also 4 mm thick, the resistance in transferring moisture contained in the moist gas or heat to the permeable membrane is considerably high. This brings about a problem in that the dew-point temperature of the dry gas cannot be raised sufficiently.